I came across this string stretcher device.
It's called The Stretcha.
It claims to properly and evenly stretch new strings and maintain tuning.

I never really made a point to properly stretch newly strung strings. Can anyone comment on the practice of stretching strings and or the relevance of such a device?

Apparently:
"A lot of musicians mistake their guitar tuning problems as a guitar problem. More often than not, the guitar is not the problem. Tuning problems are likely the result of strings that have not been stretched properly."

+1 to no device needed. I wrap a few fingers under the string around the 12th fret, tug on it pretty hard a few times, lifting it well off the fretboard, release and repeat for the other strings. Then tune it back up to pitch. Usually takes 3-4 iterations before it stabilizes and hauling the strings no longer pulls it out of tune.
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Ian C.Feb 16 '11 at 22:46

I agree. String the guitar, pull them away from the fretboard a bit to make certain they're firmly attached to the posts, then strum away for a while; that'll stretch 'em out.
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neilfeinFeb 16 '11 at 22:55

2

Just a note here... You should stretch over the length of the string, not just in one spot. I used to be in the pull-on-the-string-at-the-12th-fret club and I found that it a) doesn't do the job completely and b) the strings break more often around the pulled area. Use your fingers to do what the string-strecha does along the whole length of the neck and you'll be fine.
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AnonymousFeb 18 '11 at 23:39

@Anonymous that is the best method. There's no 'rinse and repeat' required as there is with the top answer.
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FergusApr 3 '14 at 21:12

I don't have the reputation to comment on DRL's answer (but I can and have upvoted it), but it describes exactly the method I've used for almost 40 years with no problem on electric and acoustic guitars. On my classical guitar, I don't pull the strings; I just retune several times until the strings stretch themselves out. No tools required.

Really don't think buying a string streching device is necessary but streching them out by hand does help with the tuning stability of new strings. I usually restring my guitar to pitch then go up and down each string, pressing down on the string with my thumb and pulling up with my index and ring finger a few times (holding the string down at the nut with your other hand usually helps it from popping out). Then i retune the guitar and repeat the process again two more times.

I usually tune the strings higher (about 1 semitone) than the usual tuning. Then I stretch them a bit by pulling them gently but firmly at the 12th fret, away from the fingerboard. After an hour, I check again - by that time, they're usually too low, so I tune them once again about 1 semitone over the usual tuning etc.

With this method, I find that my strings find their tune within about one day.

I just noticed, that I used the phrase "gently but firmly" "away from the fingerboard", and the accepted answer (which was written earlier) uses "slowly but firmly" "away from the fingerboard". Sorry, this partial plagiarism was unintentional... We both seem to do this quite similarly - I just found out someday, that tuning higher than usual makes the process significantly faster.
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Chris LercherFeb 17 '11 at 9:57

I agree with all the above answers, no device is required to stretch the strings. I use a couple of fingers to hold the string I'm stretching; for the thinnest strings I'll use a clean cloth as well, to spread the pressure of the string on my fingers.

Grip the string around the 12th to 15th fret and pull the string perpendicular away from the neck and body. If the guitar is laying on its back, you'll be pulling up; if in playing position, pull away from your body. I use the hand I'm not pulling with to make sure the string I'm stretching doesn't leave its slot in the nut. Also, remember to wipe the string clean after you're done stretching it.

I mostly play electric guitars, and I will stretch new strings extensively when installing them. I usually stretch metal acoustic guitar strings with less force on each stretch, but more passes of stretching and retuning. This is because I worry about putting extra tension perpendicular to the guitar top on acoustic guitars.

On electric guitars with tremolo systems, stretching one string will pull the bridge and lower the tension (and pitch) of all the other strings. I'm usually able to hold larger trem units (Floyd Rose and related designs) in place with my elbow while stretching the string, but not strat-style trem bridges. One can also block the bridge, or use some other method to hold it stationary while stretching one or more strings.

In any case, after stretching one string to the point where it stops losing pitch to the stretch, check the tuning of the rest of the strings. Even on a fixed bridge electric, they might have gone slightly out of tune. Tune them all up, and move on to the next string to be stretched.

After I get all my new strings stretched, I'll tend to play for several minutes, trying to bend notes on all the strings. Usually by this point they won't need any further stretching, but sometimes it can take several iterations to get a new string to settle in. Just keep stretching and playing until you're satisfied with how the new strings are holding their tune.

On an electric I tune the strings about right, then pull on the whammy bar, maybe getting 3 semitones up a couple of times and then tune in again. On a Floyd Rose I'd do this the next day as well and then they'll be in for good, and on non-Floyd Rose's they'd be fine with just this.

On an acoustic I'd just tune the strings all about a semitone high then after a good strum I'd tune them right. Seems to work for me.

Quality strings do not require stretching, and when an attempt is made to stretch them, they return to the original pitch when released.

Strings that stretch are not made of the proper string material known as spring steel or a facsimile thereof.

Not returning to the original pitch indicates that the material has been extended beyond its elastic limit. It does not have the properties of spring steel, and so is well suited for instrument strings.

Imagine if the springs in a car's suspension did that. Eek! You'd drive it out of the dealer's lot in an "off road" setup, and within a day you'd have a "low rider".

I find that Ernie Ball Super Slinky strings are very good in this regard. Not perfect, but very good. They are good enough that on a guitar with a double locking tremolo system, I can tune up with the headstock tuning machines once, lock the nut, and then just use the fine tuners to make very minor adjustments (in spite of string bending and tremolo use). With some other brands I used in the past, the strings would go so flat from bending, that I have to open the locking nut several times to use the tuning machines, because the fine tuners would run out of range.

So the proper way to stretch strings that require stretching is to keep pulling until they snap, and then throw them away.

I don't think the proper material for classical guitar strings is spring steel; the nylon strings I've used stretch a noticeable fraction of an inch within a few days of installation before they start to be reasonably stable, and from what I understand that's typical. Having a device or means expedite the "breaking in" period would be helpful.
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supercatDec 26 '14 at 22:33